The Upper Atmospheric Research Satellite is due to re-enter Earth's atmosphere in the next 24 hours.

SPACE JUNK COMES IN different shapes and sizes, and can pose two main kinds of threats—a threat to other spacecraft (unmanned and manned) through collisions, and threats to us down here on Earth.

The satellite making news at the moment—the former Upper Atmospheric Research Satellite, better characterised as a decommissioned or defunct satellite rather than space junk—falls into the second category.

In 2005, NASA decommissioned UARS and intentionally placed it into an orbit a couple of hundred kilometres lower than its operational orbit. This was done to accelerate is eventual demise, and means it is re-entering the atmosphere 20 years earlier than it otherwise would have done.

This was a very responsible thing to do. The longer a spacecraft stays in orbit, the more chance it has of being hit by other orbital debris, leading to a destructive breakup and therefore many more bits of debris.

UARS poses a negligible threat to life and property on Earth. Most of the satellite will burn up during re-entry, with perhaps as many as 26 of the stronger or harder small pieces surviving to reach the surface.

But with the majority of Earth comprising oceans or uninhabited (or very sparsely inhabited) remote regions, the chances are overwhelming that any pieces of UARSthat survive re-entry will fall harmlessly and never be seen again.

This map shows the orbital path of the Upper Atmospheric Research Satellite, and it's predicted impact located (yellow symbol within the orange circle at left) in the southern Pacific Ocean.

Because the spacecraft is no longer powered, NASA has no control over where it comes down.

It is thought to be tumbling gently as it makes its final orbits. Friction with the thin upper atmosphere is slowly lowering its orbit, bit by bit. Sometime in the next 24 hours it will reach a low enough point and sufficient air friction such that it will no longer be able to maintain orbital velocity.

At this point it will begin to burn up and streak across the sky like a huge fireball. It would be quite something to see, but chances are that no one will witness it.

The other kind of space junk—bits of orbital debris that range from less than a millimetre wide up to entire spacecraft—is more of a worry. Space junk can damage or destroy an operational spacecraft, leading to loss of the asset and the service it provides.

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Hi Victor.
UARS was solar powered, not nuclear.
This satellite was not designed (back in the 1980s) with pinpoint re-entry in mind. It, and many others, are intended to make uncontrolled re-entries. Controlled re-entries require extra systems and fuel etc, which adds to the cost.
For those spacecraft that have plutonium power sources, the plutonium is encased in an incredibly tough material that *is* designed to withstand the heat of re-entry and impact with the ground, just in case something goes wrong. See http://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator#Safety
Cheers,
Jonathan

Pondering how the americans can and have previously pinpointed re entry sites but suddenly cant do it with this satellite.
Also I understand that it was powered by nuclear source, and if so surely will pose threat to people etc from the plutonium source, which would not be incinerated on re entry????